1. Field of the Invention
This invention relates to light metering devices of the TTL type for use in exposure control systems, and more particularly to the compatible arrangement of such light metering device with a range finder of the TTL type by using an amorphous silicon photo-diode (hereinafter abbreviated as "ASP" diode).
2. Description of the Related Art
In single lens reflex cameras, it has been the general practice that the sensor of the light meter is positioned either on the exit face of the pentagonal roof type prism of the finder system, or in the mirror box. This causes a drawback that no light value proportional to the F-number of the objective can be obtained, since the sensor is far away from the focal plane of the objective.
An attempt has been made to put the sensor at the side of the focusing screen by using a beam splitter of very small size positioned adjacent the screen to direct a splitoff portion of the finder light sideway to the sensor. Even in this case, a defect due to the long distance between the focal plane and the sensor is encountered.
Also, for an automatic focusing system built in such a camera, it is a prerequisite that the arrangement of an image sharpness sensor be made without adversely influencing the measurement of the brightness level of an object to be photographed. The optics for detecting the state of focus of an image formed by the objective, which is very suited to the single lens reflex camera, is to split the pupil of the objective into two parts from which two respective beams are obtained to form two second images of the same object behind a predetermined focal plane of the objective. By detecting the relative positions of these two images with respect to each other, the state of focus of the objective can be determined. Such an optics or so-called re-focusing system is proposed in Japanese Laid-Open Patent Application No. SHO 52-95221.
To install this focus detector in the single lens reflex camera, the presence of a limitation on the optical path length leads, in most cases, to making a central portion of the area of the quick return mirror semi-transparent and directing the transmitted light to the focus detector on the bottom of the mirror box.
Similar to this arrangement, light metering at the focal plane has been known where the light emerging from the semi-transparent area of the quick return mirror is focused to the sensor on the bottom of the mirror box, giving an advantage that the accuracy of measurement of the object brightness is improved while nevertheless hardly affecting the image on the focusing screen.
The simultaneous occurrence of both such light arrangements for the automatic focusing and light metering systems, however, will result in a very complicated structure of the bottom portion of the mirror box, which will in turn cause an objectionable increase in the size of the entire body of the camera.
Meanwhile, since a technique of fabricating ASP diodes on a substrate by vacuum evaporation has recently been developed, the use of such diodes for light metering is being proposed because they can be built in the camera without taking a large space which is advantageous.